This invention relates to a system for determining the condition of a patient""s heart.
The use of electrocardiographic (ECG) information to detect ischaemic heart disease (IHD) is not new. The standard twelve lead ECG has been used by clinicians for decades for early detection of ischaemic events. The procedure involves the use of 10 appropriately placed electrodes and suitable instrumentation amplifiers to acquire 12 separate ECG signals. These signals are then interpreted either visually or by automated software to identify ischaemic signs. Unfortunately, the area of the torso covered by these 10 electrodes is insufficient to detect ischaemic events from all areas of the heart. This means that the standard twelve lead ECG in many instances fails to provide unequivocal diagnosis.
An improved method is the use of unipolar body surface mapping (BSM) which uses a multitude of electrodes (typically between 32 and 200 electrodes) distributed across both the anterior and posterior surface of the torso. With such a system the amount of information being presented to the user is impractical. Furthermore, with large numbers of electrodes, the system requires a significant amount of time to be applied to the patient. One approach of particular interest is by Lux et al. xe2x80x9cRedundancy reduction for improved display and analysis of Body Surface Potential Maps I spatial compressionxe2x80x9d, Circulation Res, Vol. 49, 186-196; where a Karhunen-Loeve method is described which allows a minimal lead set to be recorded and then later expanded mathematically to a more detailed lead set.
Investigations concerning the analysis of such BSM""s both directly recorded and expanded mathematically has resulted in various different analysis techniques. All of these however are essentially enhancements to the analysis techniques used to interpret the standard 12 Lead ECG. BSM information is unique in that it provides an overall body surface electrical pattern. This pattern is distinctive and must be analyzed in a way which takes advantage of the information contained within it.
The use of vectors in ECG interpretation is known, the most famous being Vectorcardiographic systems which are no longer common. All of these vector analysis techniques however concentrate upon the discrete amplitude of the vector drawn between a maximum and a minimum point of electrical potential. One such system is disclosed in European Patent Specification EP-A-0512 719 B1 where a system is described for detecting coronary artery disease by use of a discriminant function. Here one such parameter which could be analyzed is the overall QRST vector. This would be a vector drawn between the maximum and the minimum point of a QRST isointegral BSM.
The use of discriminant functions as described above has been for many years arguably the best method for analyzing the parameters and features extracted from BSM""s. One notorious problem with a discriminant function approach is the lack of determinism associated with such a technique. Given any particular case or set of parameters it is very difficult to see what output a given function will provide and given an output it is very difficult to determine how the function arrived at that decision.
The use of a more conventional decision tree approach has not been considered appropriate since the problem possesses so many dimensions. Having obtained a decision node (using binary comparison of a given parameter to a preset threshold), which will reliably and accurately detect one given patient condition, it is later found that the same decision fails when complicated by other real life conditions. For example, having devised a decision tree which can be useful in detecting acute myocardial infarction occurring in all areas of the myocardium, this same algorithm then fails when there are two areas of the myocardium infarcting at the same time, when the heart is abnormally shaped due for instance to hypertrophy or when the infarct is complicated by a disorder of the conduction system.
It is an object of the present invention to provide a means for analyzing cardiac information which can provide an improved diagnostic capability with a clearly traceable path to the resulting decision.
Accordingly, the invention provides a system for determining the condition of a patient""s heart, comprising:
(a) a plurality of electrodes each capable of detecting the electrical activity associated with a heartbeat of the patient and producing a corresponding cardiac signal,
(b) means for converting the cardiac signals into digital form, and
(c) data processing means programmed to:
(1) process the digital cardiac signals to determine a plurality of parameters of the patient""s heartbeat,
(2) determine the condition of the patient""s heart using a binary decision tree algorithm, such algorithm having a plurality of decision nodes each of which makes a decision based upon the value(s) of a respective subset of the parameters, the decision criterion of at least one of the said decision nodes being modified according to the value of at least one parameter not of the respective subset, and
(3) provide an output indicative of the condition of the patient""s heart so determined.